A current sensor has been proposed that detects an electric current flowing through a shunt resistor based on a resistance of the shunt resistor and a voltage drop across the shunt resistor. For example, this type of current sensor is used to detect charging and discharging currents of a vehicle battery, or an excitation current of an electric motor installed in an electric hybrid vehicle (EHV) and an electric vehicle (EV).
A shunt resistor disclosed in JP-A-H8-115802 includes L-shaped legs and has a low temperature coefficient. The shunt resistor is surface-mounted on a printed circuit board by soldering the L-shaped legs to the printed circuit board. A resistor member disclosed in JP-A-H9-213503 includes a molded conductor having exposed end portions that are plated to act as contacts. A current sensor disclosed in U.S. Pat. No. 4,675,255 includes a contact attached to a battery terminal of a vehicle, a shunt resistor electrically coupled to the contact, a detection circuit for detecting an electric current flowing though the shunt resistor, a pole body for covering the contact, and a housing that is unitary with the pole body and accommodates the shunt resistor and the detection circuit. These prior arts have the following problems:
Regarding the shunt resistor disclosed in JP-A-H8-115802 and the resistor member disclosed in JP-A-H9-213503, the shunt resistor and the resistor member are mounted on the printed circuit board through solder. Therefore, an open-circuit may be caused by a cold solder, or a short-circuit may be caused by a solder whisker. Further, since a typical printed circuit board has a low thermal resistance of about 110 degrees Celsius (° C.), the shunt resistor and the resistor member cannot be used under high temperature conditions, and cannot be used to detect a large current. For example, the shunt resistor and the resistor member cannot be used in an engine room, where the temperature increases up to 150° C., and cannot be used to detect the battery charging and discharging current, which reach about 1000 amperes (A). Furthermore, a detection circuit for detecting the current is constructed with multiple circuit elements mounted on the printed circuit board by the solder. Therefore, many manufacturing steps and cost are required to solder the circuit elements to the printed circuit board, and the problems due to the cold solder and the solder whisker may occur.
Regarding the current sensor disclosed in U.S. Pat. No. 4,675,255, space between the battery and a hood (i.e., bonnet) of the vehicle is required, because the contact is attached to the battery terminal of the vehicle. Further, the contact is attached to the battery terminal in a specific manner. Therefore, it is likely that the contact is incorrectly attached to the battery terminal during replacement of the battery. Further, the housing is not fixed to the battery, whereas the pole body, which is unitary with the housing, is fixed to the battery. Therefore, a large force is applied to a joint portion between the housing and the pole body and damages the joint portion.